1. Field of the Invention
This invention is related to the field of ultra-short pulsewidth lasers, and particularly to apparatuses and methods for performing temporal scanning with minimal (i.e., micron-scale) mechanical movement. The invention also relates to methods used for obtaining high-accuracy (i.e., sub-picosecond) timing calibration, applicable to the above-mentioned temporal scanning methods or to conventional temporal scanning methods. In particular, the invention eliminates the need for a mechanical scanning delay arm in a correlator or other type of pump-probe device, including ranging, 3-D imaging, contouring, tomography, and optical time-domain reflectometry (OTDR).
2. Description of the Related Art
Ultrafast laser oscillators are presently known which are capable of generating pulsewidths of the order of tens of femtoseconds with nanojoule-level pulse energies, at repetition rates ranging from 5 MHz to as high as 1 GHz. Such short pulses are used for many applications including measurements by time gating, including metrology. Many applications of such short optical pulses require that one set of optical pulses be delayed with respect to another set of optical pulses, in which temporal delays must be known to a very high accuracy, such as on the order of 10 femtoseconds. Temporal delays for short pulses find many uses in such applications as biological and medical imaging, fast photodetection and optical sampling, optical time domain reflectometers, and metrology.
The conventional method for delaying and scanning optical pulses is to reflect the pulses from a mirror and to physically move the mirror, using some mechanical means, by a distance D, which is defined by the product of the time delay, ΔT, and the speed of light in vacuum c=3.0×108 meters/sec. Thus:D=c/2×ΔT or D(cm)=15×ΔT(nsec).This type of delay will be termed here a physical delay. Also, scanning, as that term is used here, refers to the systematic changing of the difference in time of arrival between two optical pulses. Various methods and devices have been developed to provide the accurate positioning and scanning of the mirror, involving:                Voice-coil type devices (shakers) (R. F. Fork Nd F. A. Beisser, APPL Opt. 17, 3534 (1978)).        Rotating mirror pairs (Z. A. Yasa and N. M. Amer, Opt. Comm., 36, 406 (1981)).        Linear translators employing stepper motors, which are commercially available from many vendors.        Linear translators employing galvanometers. (D. C. Edelstein, R. B. Romney, and M. Scheuermann, Rev. Sci, Instrum. 62, 579 (1990)).Other types of physical delays use adjustable group delay including:        Femtosecond pulse shapers (FPS) employing scanning galvanometers (K. F. Kwong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A. Dienes; “400-Hz mechanical scanning optical delay line” Opt. Lett. 18, (7) 558 (1993) (hereinafter Kwong et al.); K. C. Chu, K. Liu, J. P. Heritage, A. Dienes, Conference on Lasers and Electro-Optics, OSA Tech.Digest Series, Vol. 8, 1994, paper CThI23.).        Rotating glass blocks.        
The physical delay methods suffer from a number of disadvantages, the chief one being the large space required if large delays are desired. For example, a delay of 10 nsec requires a mirror displacement of 5 feet. There are other physical limitations and disadvantages as well. Misalignment and defocusing can distort measurements when large delays are used. Using corner-cube retroreflectors reduces the problem of misalignment, but not defocusing. The defocusing effect can occur when the scan amplitude is an appreciable fraction of the confocal parameter of the beam. A time delay of 10 nsec entails a change in free-space propagation of 10 feet (˜3 meters). Thus, to minimize the effects of defocusing, the confocal parameter (ZR) must be approximately 10-times this number or ZR=30 meters. At a wavelength of 1550 nm, this requires a beam radius of wo=12 mm. This is impractically large for many situations.
The need for large mirror displacement can be reduced by multi-passing the delay line (e.g., double-passing the delay line cuts the required mirror displacement in half), however, this does not alleviate the defocusing problem. Multipassing causes its own set of problems in that alignment procedures are more complicated, and the optical losses are increased.
Yet another limitation has to do with the scanning rates and scanning frequencies which can be achieved simultaneously. It is often desirable to signal average while scanning rapidly (>30 Hz) in order to provide “real-time” displays of the measurement in progress. However, the scanning range is limited at such high scan frequencies. The best achieved scan range is 100 psec at a rate of 100 Hz using the scanning FPS method (Kwong et al.). Any further increase of the scanning range and/or frequency with such reciprocating devices would cause high levels of vibration, which can be disruptive to laser operation. Rotating glass blocks avert the vibration problem, and are capable of higher scan speed, but lack any adjustability of scan range, and they introduce variable group velocity dispersion which makes them inappropriate for use with pulses having widths shorter than 100 fsec.
In addition to the physical delays, methods have been introduced which provide temporal scanning without the need for any mechanical motion. These include:                Free-scanning lasers (A. Black, R. B. Apte, and D. M. Bloom, Rev. Sci, Instrum. 63, 3191 (1992); K. S. Giboney, S. T. Allen, M. J. W. Redwell, and J. E. Bowers; “Picosecond Measurements by Free-Running Electro-Optic Sampling.” IEEE Photon. Tech. Lett., 6, pp. 1353-5, November 1994; J. D. Kafka, J. W. Pieterse, and M. L. Watts; “Two-color subpicosecond optical sampling technique.” Opt. Lett., 17, pp. 1286-9, Sep. 15, 1992 (hereinafter Kafka et al.); M. H. Ober, G. Sucha, and M. E. Fermann; “Controllable dual-wavelength operation of a femtosecond neodymium fiber laser,” Opt. Lett. 20, pp. 195-7, Jan. 15, 1995).        Stepped mirror delay lines employing acousto-optic deflectors as the dispersive elements (R. Payaket, S. Hunter, J. E. Ford, S Esener; “Programmable ultrashort optical pulse delay using an acousto-optic deflector.” Appl. Opt., 34, no. 8, pp. 1445-1453, Mar. 10, 1995).        Slewing of RF phase between two modelocked lasers (D. E. Spence, W. E. Sleat, J. M. Evans, W. Sibbett, and J. D. Kafka; “Time synchronization measurements between two self-modelocked Ti:sapphire lasers.” Opt. Comm., 101, pp. 286-296, Aug. 15, 1993).        
The non-mechanical methods, in particular, are capable of high speed scanning. The free-scanning lasers produce a scan range which spans the entire repetition period of the laser. For example, a known free-scanning laser system is shown in FIG. 1, which includes a master laser 10 and a slave laser 20 having different cavity lengths which produce pulse trains at different repetition frequencies ν1 and ν2. The scan frequency is equal to the frequency difference Δν=ν1−ν2, and is set at the desired value by adjusting the cavity length of the slave laser to a specific fixed length. A correlator 40 produces a signal from the cross-correlation between the two lasers which gives information about the timing between the two lasers, and provides triggering signal to data acquisition electronics 50. For example, in Kafka et al. two independent, mode-locked Ti:sapphire lasers, namely, master laser 10 and slave laser 20 each with a nominal repetition rate of 80 MHz, were set to have different repetition frequencies (by about 80 Hz). Due to the offset in repetition frequency, the lasers scanned through each other at an offset frequency Δν of approximately 100 kHz. This offset frequency can be stabilized to a local RF oscillator. Since the laser repetition rates were near 80 MHz, the total scan range was about 13 nsec. Thus, time scanning was achieved without employing any moving mechanical delay lines. Timing calibration was achieved by cross-correlating the two laser beams, reflected off of a mirror 30, in a nonlinear crystal, i.e., correlator 40, the resulting signal being used to trigger data acquisition unit 50 (e.g. an oscilloscope). The laser beams output from lasers 10 and 20 are also reflected off another mirror 60 and received by a measurement apparatus 70 which performs a desired measurement or experiment using the laser beams.
The chief drawback to this technique is that it is highly wasteful of data acquisition time for two main reasons:                1. Fixed scan range—The scan range is fixed at the inverse of the repetition frequency (i.e., the round trip time) of the laser.        2. Dead time—One is often interested in only a 100 psec, or 10 psec scan range instead of the full 13 nsec pulse spacing. Thus, only 1% (or 0.1%) of the 10 μsec scan time is useful, while the remaining 99% (99.9%) is “dead-time.” This increases data acquisition time by a factor of 100 or 1000.        
Kafka et al. address these limitations and suggest that this can be partially circumvented by using lasers with higher repetition rates, (e.g., νo=1 GHz). However, this solution is unacceptable for many applications which require a large variety of scan ranges. For example, pump-probe measurements of semiconductors are frequently conducted over a large variety of time ranges. Lifetimes of carriers (i.e., electrons and holes) of the semiconductor are on the order of several nanoseconds which makes a 1 GHz laser completely unacceptable, since residual carriers from the previous laser pulse would still be present when the next pulse arrives. Yet at the same time, it is often desirable to zoom-in on a much narrower time scale (e.g., 50 psec) to look at extremely fast dynamics. Thus, the free-scanning laser technique lacks the versatility of scan range selection which is required in many applications. The way to get a large temporal dynamic range without having extremely long acquisition times is to have the flexibility of a coarse and fine timing adjustment.
In related work, several methods have been used to stabilize the timing between two modelocked lasers, in cases where the lasers were actively mode locked, passively modelocked, and regeneratively modelocked, or with combinations of passively and actively mode-locked lasers. The methods used for synchronization can be divided into two main types: (1) passive optical methods; (2) electronic stabilization. The highest synchronization accuracy is achieved by passive optical methods in which the two lasers interact via optical effects (J. M. Evans, D. E. Spence, D. Bums, and W. Sibbett; “Dual-wavelength selfmode-locked Ti:sapphire lasers.” Opt. Lett., 13, pp. 1074-7, Jul. 1, 1993; M. R. X. de Barros and P. C. Becker; “Two-color synchronously mode-locked femtosecond Ti:sapphire laser.” Opt. Lett., 18, pp. 631-3, Apr. 15, 1993; D. R. Dykaar, and S. B. Darak, “Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser,” Opt. Lett., 18, pp. 634-7, Apr. 15, 1993 (hereinafter Dykaar et al.); Z. Zhang and T. Yagi, “Dual-wavelength synchronous operation of a mode-locked Ti:sapphire laser based on self-spectrum splitting.” Opt. Lett., 18, pp. 2126-8, Dec. 15, 1993). These optical effects, (e.g., cross-phase modulation) cause a rigid locking effect between the two lasers which become synchronized to within less than one pulse width (<100 fsec). Although these give the most accurate synchronization, the time delay between the lasers is rigidly fixed; so that in order to scan the time delay between them, one must use the conventional physical scanning delay methods.
Electronic stabilization using simple RF phase detection gives the most flexibility in terms of adjusting the relative time delay, but at the present time these systems can maintain timing accuracy of no better than a few picoseconds (˜3 psec). Such a system is commercially available for stabilizing a Ti:sapphire laser to an external frequency reference, or for synchronizing two mode-locked Ti:sapphire lasers. (Spectra Physics Lok-to-Clock™ system). Stabilization of better than 100 fsec has been achieved by use of a pulsed optical phase locked loop (POPLL). This is a hybrid opto-electronic method, as disclosed in S. P. Dijaili, J. S. Smith, and A. Dienes, “Timing synchronization of a passively mode-locked dye laser using a pulsed optical phase locked loop.” Appl Phys. Lett., 55, pp. 418-420, July 1989 (hereinafter Dijaili et al.), in which the electronic stabilizer circuit derives the timing error signal from an optical cross-correlator. However, this method suffers from the same lack of timing adjustability as the passive optical methods. The timing can only be adjusted by less than one pulse width. Thus, using the POPLL method, it would be necessary to insert some sort of physical delay line into one laser beam if it were desired to change the relative pulse timing by anything more than one pulsewidth.
The performance of timing stabilization by RF methods could improve if the intrinsic timing jitter of the laser can be reduced. Some reduction of intrinsic laser jitter can be achieved by insuring that the two lasers are exposed to identical environmental conditions as much as possible. The Sticky-Pulse laser, which is disclosed by Dykaar et al., employs a spatially split pump laser beam to pump two spatially separate regions of a Ti:sapphire laser crystal. This is essentially two separate lasers which share the same pump laser, laser crystal, air space, and most other intracavity elements except for the two end mirrors. In this way, the two lasers experience the same thermal fluctuations, pump laser noise, and turbulence, thus minimizing the difference in repetition rate jitter. This allows even a weak optical interaction between the two lasers to lock the pulses together. This general principle of “environmental coupling” can be applied to other types of lasers including mode-locked fiber lasers. However, it should be noted the object of Dykarr et al. is to lock the two lasers together, which is undesirable for the purposes of the present invention, because then the time delay cannot be scanned; i.e., the timing of the pulses from the two coupled lasers of the Sticky-Pulse laser discussed above, are locked together through the optical coupling and they cannot be independently controlled.